The present invention relates to a method for reducing interference used in a radio system including several base stations and subscriber terminal equipments which comprise a transmitter and a receiver, and which use a spread spectrum method for sending a signal, in which method a narrowband signal is spread by a spreading code into a broadband signal.
The present invention also relates to a radio system comprising several base stations and subscriber terminal equipments which comprise a transmitter and a receiver, and which are arranged to use a spread spectrum method for sending a signal, in which method a narrowband signal is spread by a spreading code into a broadband signal.
In the spread signal system and particularly in the CDMA technique, a user""s narrowband data signal is modulated by a spreading code with a broader band than a data signal to a relatively broad band which is usually over 1 MHz. Generally, a spreading code is formed of a long pseudo-random bit sequence. The bit rate of a spreading code is considerably higher than that of a data signal and in distinction from data bits and data symbols, bits of a spreading code are called chips. Each data symbol of a user is multiplied by all the chips of the spreading code. In that case a narrowband data signal will be spread to the frequency band used by the spreading code. Signals of each user has a specific spreading code. When several users send their signals simultaneously on the same frequency band, data signals are separated from one another in receivers on the basis of a pseudo-random spreading code.
The bit sequences of spreading codes used in the CDMA system are not, however, completely independent of one another, that is, orthogonal.
Non-orthogonality becomes stronger if the sequences have shifts with respect to one another, and this non-ideal state results in that the cross correlation between different codes varies at different delay values. Because of this, the mutual interference of signals from a different radio system vary in multipath propagation when signal routes and thus delays change, particularly when subscriber terminal equipments move. In that case in interference cancellation based on the correlation of spreading codes, a correlation matrix has to be continuously updated, which is laborious to calculate. Furthermore, when the delay with high data rates is of the length of the code sequence, that is, of the data symbol, it causes mutual interference between signals. Because of varying delays, the selection of the spreading codes to be used is difficult when low cross correlation values are to be attained.
Present radio systems do not use interference cancellation, which is followed by that the capacity of the systems will remain relatively low and the quality of connections will suffer. The number of useful spreading codes will also remain rather low as the cross correlations between the spreading codes has to be low on all delay values.
The object of the present invention is to realize a method with which the capacity of the radio system is enlarged, the number of useful spreading codes is increased and the quality of the connection is improved. The amount of calculation needed for interference cancellation can also be reduced.
This is attained with a method shown in the preamble that is characterized in that the receiver of the signal forms mutual interference information on the signals received by it and on the basis of the mutual interference information, the receiver sends to the transmitter of the signal a control command of the spreading code of the signal comprising information about the size of the phase control needed in the phase setting of the spreading code, the receiver of the signal similarly changes the phase setting of its spreading code used for reception, and the transmitter of the signal adjusts the phase of the spreading code used for transmitting the signal on the basis of the control command sent from the receiver of the signal to diminish mutual interference between different signals.
The method of the invention is also characterized in that the receiver of the signal sends to the transmitter of the signal a control command of the spreading code of the signal comprising information about phase hopping, the receiver of the signal similarly hops the phase of its spreading code used for reception, and the transmitter of the signal adjusts the phase of the spreading code used for transmission by hopping the phase of the spreading code continuously on the basis of a predetermined sequence to diminish mutual interference between the signals.
The radio system of the invention is characterized in that the receiver of the signal comprises an interference information means for forming mutual interference information on the signals received by it, the receiver comprises a command means arranged to form a control command of the phase of the spreading code on the basis of the mutual interference information formed by the interference information means and to send in the control command information about the size of the phase adjustment of the spreading code, or about the sequence of spreading code hopping, and the receiver of the signal is arranged to adjust the phase of the spreading code of its reception on the basis of the control command, and the transmitter of the signal comprises a control means for adjusting the phase of its spreading code on the basis of the control command sent from the receiver of the signal.